v0.3.0

New addin: scribble! Now you can draw on your slides! Huge thanks to
@mattwarkentin for developing and contributing this extension. With
use_scribble() you can draw on your slides using your mouse or a tablet with
stylus. Drawings stay with each slide and scale when the slides are resized.
You can erase individual lines, undo or redo with keyboard shortcuts, or clear
the drawings on the current slide easily with the provided toolbox.
Thanks also to @LauraRK and @rpruim for suggestions and feedback.
(@mattwarkentin #87)